UPDATE: Steppes has now been re-added to the map cycle as of August 2015. When I was going for the achievement it wasn't even in the cycle.. which is why I never found Steppes.
The biggest key to getting this is knowing how to even find the maps!!
This should also apply to Steppes theoretically: After many games I found that Tier's 5 and 6 gave me the best chance of finding Highway, although I once got Highway with a Tier 4 Artillery. I didn't try out Tier 7 and above simply because of the silver, time, and not having very many of them..However, it's really frustrating trying to get these maps as it is so random.
I alternated between 4-5 different Tier 5 and 6 tanks while trying to get the right maps.
Highway is a rather open map. Steppes apparently is even more open with slight hills and looks like fun.
I found Highway eventually many times, but getting the FIRST kill is even worse than scoring in the top 3 for XP!! Of all things, I ended up getting it with an Artillery.
I was going for the French Artillery "Trebuchet!" achievement when I got highway several times with my Tier 5 arty. I would just blind-fire at the enemy spawn, but never hit anything, I was hoping to hit an AFK or not yet spawned enemy.
I'm including the video of the kill that got the achievement just so you have a brief idea of how I did it, maybe that will help someone somehow!
One other thing--I found this map many times with some US Tier 5 and 6 tanks. I just never managed to get the first kill as I'd either rush too fast, or some silly speed demon on the other team would get himself killed across the map before I could get there. I recommend looking where your team goes, and then try to get ahead of the majority crowd.
Enemy movements in Standard battles on Highway--
South-west corner enemy spawn: I found they normally NEVER mass-pushed through the middle. Almost always though a scout or two would flank the left side. Their main push usually went straight to the town for some reason, and was terrible for arty fire.
North-east corner spawn: Weird, but usually always some tanks would go into the valley/creek through the middle, some always to the west side and then south, and a few to the town who usually got outnumbered(lol).
Please comment with ANY ways you managed to get this achievement faster!! It was a major pain and any help for others would be really appreciated.